1. Field of the Invention
The present invention relates to methods for producing containers having excellent barrier properties.
2. Description of the Related Art
Containers excellent in barrier properties have heretofore been used for containers for food packaging, beverage packaging, medical product packaging, etc. Due to excellence in rust prevention, lightweight property, shaping workability and design flexibility, etc., barrier containers made of thermoplastic resin are in wide use as fuel containers in the automobile field. Examples of barrier fuel containers made of resin which are generally used include multilayer containers comprising a layer of high density polyethylene and a barrier layer made of an ethylene-vinyl alcohol copolymer. U.S. Pat. No. 6,033,749 discloses a fuel container having improved gasoline barrier property, especially oxygen-containing gasoline barrier property, the fuel container being multilayer container comprising a high density polyethylene layer, an adhesive resin layer, an ethylene-vinyl alcohol copolymer layer, an adhesive resin layer and a high density polyethylene layer which are arranged in this order, wherein the ethylene-vinyl alcohol copolymer layer, which serves as a barrier layer, is disposed closer to the inner surface.
Such a fuel container disclosed in U.S. Pat. No. 6,033,749, however, has a problem that since the innermost layer of the fuel container is a high density polyethylene layer, this layer is adversely swollen with a fuel in the container. This problem has obstructed such fuel containers from being recycled after use.
As a solution to such problems, US 2002/0176955 A1 discloses a fuel container comprising a container body obtained by joining an upper section and a lower section each made of a thermoformed multilayer sheet, wherein the inner surface of the container body has been covered with a barrier material. This document also discloses a method for producing the fuel container wherein the method comprises thermoforming two multilayer sheets to obtain two thermoformed multilayer sheets, covering a surface of each of the thermoformed multilayer sheets with a layer of a barrier material, where the surface is to define the inner surface of a container when the thermoformed multilayer sheets are joined together at their edge portions to form the container, and heat sealing the edge portions of the thermoformed multilayer sheets.
US 2002/0176955 A1 teaches that there is a need to fail to cover the portions to be heat sealed of multilayer sheets when covering the surfaces of the multilayer sheets with a barrier material. This is because if the portions to be heat sealed are covered with the barrier material, the adhesion strength in the sealed region resulting from heat sealing would be reduced. On the other hand, from the viewpoint of barrier properties, it is desired that the inner surface of a container be covered with a barrier layer close to the sealed portion. As described above, conventional technologies may be inferior in workability because a barrier material should be applied carefully in order to cover the inner surface with a barrier material completely without applying the barrier material to portions to be heat sealed.